Construction machines are frequently used by its nature in a place whose road surface is in a bad condition as compared with general vehicles. However, on a soft ground such as a mine or a construction site, since positional differences between the wheels lead to different friction coefficients between the wheels and the road surface, even the construction machines suffer from the slip of part of the driving wheels, so that a driving torque cannot be transmitted to the other driving wheels. In such a case, most of engine output is used to drive the slipping driving wheel or wheels, so that a sufficient amount of the driving force cannot be transmitted to the road surface, thereby reducing acceleration. In view of the above, it is required to control the driving torque to each wheel in accordance with a road surface condition so as to change the amount of the driving force transmitted to the road surface from the wheel to be suitable for the road surface.
As a device for controlling the driving force to each wheel of the construction machines, there has been known a traction control (hereinafter referred to as TCS) device that automatically applies brake to each wheel, thereby adjusting a driving torque to the wheel (see, for instance, Patent Literature 1).
Generally, the TCS device detects the rotation speeds of wheels and estimates the vehicle speed of a construction machine from the detected rotation speeds. The TCS device finds the slip condition of each wheel with reference to the estimated vehicle speed and the rotation speed of the wheel and applies brake to the wheel in accordance with an excess of an actual slip ratio relative to a target slip ratio.